1. Field of the Invention
The present invention relates to an apparatus for operation on a cornea, and more particularly to the apparatus which corrects ametropia by ablating a corneal surface with a laser beam, being suitable for correcting simple myopic astigmatism and mixed astigmatism.
2. Description of Related Art
In conventional art, there is known an apparatus for operation on a cornea, which corrects ametropia of an examined eye by ablating a corneal surface with an excimer laser. Several methods have been proposed for laser irradiation for such a kind of the apparatus, to include a one shot irradiation method which covers an irradiation area of a cornea with only one shot, a scanning slit method which puts side by side with superposing a slit (rectangular) shaped laser beam in a width direction, and a scanning spot method which converges a laser beam to a diameter of 1 to 2 mm, then scans with high velocity by wagging the laser beam with a mirror having two axes of X and Y. The apparatus acting on above mentioned method enables one to perform such operation as to correct myopia and compound myopic astigmatism, further enabling one to correct hyperopia.
A typical method of correcting myopic astigmatism is to flatten the cornea in a strong principal meridian direction. One such method is to ablate the whole cornea in a strong principal meridian direction by enlarging an optical zone, and the other method is to ablate only a central part in a strong principal meridian direction selectively.
However, since a corneal surface is curved, a central part of cornea differs from that of a periphery of cornea in an energy density of a laser irradiation. Accordingly, in case of the method of correcting myopic astigmatism by ablating the whole in a strong principal meridian direction, an ablation of the periphery tends to be slight as a whole, thus resulting in such a problem that a spherical component is shifted to a hyperopic direction.
On the contrary, in the case of the method of correcting myopic astigmatism by ablating only a central part in a strong principal meridian direction selectively, since a small area in a strong principal meridian direction can be ablated, a spherical component is shifted little. However, because of a small optical zone, there is such a problem that a patient's eye is influenced by an optical disorder such as a glare or a halo.
In addition, in the case of correction for mixed astigmatism showing such symptoms as to be myopia on a strong principal meridian and to be hyperopia on a weak principal meridian, a method combines an ablation in a strong principal meridian direction with an ablation for correcting hyperopia. By combining simply, there are such problems that an ablation amount becomes much and a satisfactory result is not obtained.